Venezuela's main oil ports on Tuesday entered their fifth day without delivering crude for state-run PDVSA's customers in Asia, which are the OPEC country's main buyers, shipping data showed, as the U.S. presses the nation through an oil embargo.

Chevron, which is PDVSA's main joint-venture partner, on Monday resumed exports of Venezuelan oil to the U.S. after a four-day pause and called workers abroad back to its Venezuela offices as flights to the country restarted. The U.S. firm has emerged in recent weeks as the only company fluidly exporting Venezuela's crude.

At least a dozen vessels under sanctions that had loaded in December departed from Venezuela's waters in early January carrying some 12 million barrels of crude and fuel bound for China. They left in 'dark mode' or with transponders off, breaking a U.S. tanker blockade in effect since last month.

Washington has not clarified if it authorized those departures. PDVSA did not immediately reply to a request for comment.

An oil export paralysis to Asia could force PDVSA, which has been struggling to keep output and refining operations running, to deepen production cuts it began in recent days amid an over accumulation of crude and residual fuel inventories.